The present invention relates to a hot-top type continuous casting machine for hollow billets, which can stably cast hollow billets made of nonferrous metal such as, in particular, aluminum alloy including aluminum, with a high degree of quality, and in addition in particular to an improved core adapted to be used in a mold for casting such hollow billets.
Metal billets are used as materials for extrusion of a pipe, using a mandrel, which are advantageous since they offer a higher yield for extrusion in comparison with that of solid billets.
In order to produce the above-mentioned hollow billets, there has been used a hot-top type continuous casting machine or a direct chill casting machine.
Among them, as shown in FIG. 1, the hot-top type continuous casting machine is adapted to retain, in a molten metal holding part 2 formed of refractory materials, molten metal 3 such as aluminum alloy, which is continuously transferred from a holding furnace (not shown), through a trough 20, and the thus retained molten metal 3 is then continuously cast into a hollow billet 1 through the intermediary of a casting part 4 located underneath the molten metal holding part 2 and composed of an ring mold 6 and a core 31. This continuous casting machine in provided with a feeder head, and accordingly, the thus produced hollow billet may have a high degree of interior quality. The ring mold 6 constituting the above-mentioned casting part 4 defines the outer peripheral part of the hollow billet 1, and the core 31 defines the inner peripheral part of the hollow billet 1. The core 31 is attached to a support bar 21 located on the upper surface of the molten metal holding part 2.
The core 31 has conventionally made of aluminum alloy or graphite as disclosed in Japanese Patent Application No. S62-107749.
However, a core made of aluminum alloy has such a structure that water is injected, direct to the inner peripheral surface of a hollow billet from the lower end part of the core, and further, it is high heat conductive. Thus, a solidified shell is formed at the inner surface 5 of the ring billet on a relatively earlier stage, and this solidified shell is then remelted by molten metal in the molten metal holding part 2. Thus, the molten metal which has been remelted gives rise to an uneven surface, possibly causing leakage of the molten metal.
Meanwhile, Japanese patent Application No. S63-273553 discloses a core made of graphite, silicon nitride or the like. The graphite core has a relative low thermal conductivity so that the position where the formation of a solidified shell is initiated is likely to vary depending upon a casting condition, and accordingly, a hollow billet 1 with a high degree of quality cannot be stably produced. Further, since the outer surface of the graphite core 31 is worn with the time, resulting in ununiform cooling by the core, which would cause a defect in the inner peripheral surface of the hollow billet 1. Thus, there would be finally resulted a risk of leakage of molten metal.
Thus, there has been proposed a core having an upper part made of a refractory material and a lower part made of metal, and adapted to directly inject cooling water from the lower end part of the core onto the inner peripheral surface of a hollow billet (refer to, for example, Japanese patent No. 1630402).
However, this core is always subjected to such a risk that it is drawn into the hollow billet due to solidification contraction of the hollow part of the hollow billet. Should the core be drawn into the hollow billet, molten metal in the molten metal holding part would attack the above-mentioned injected cooling water thereover, resulting in the risk of occurrence of steam explosion. Thus, this core has not yet been practically used.
Accordingly, an object of the present invention is to provide a hot-top type continuous casting machine which can stably cast a hollow billet having a high degree of quality, and in particular, to provide an improved core used in an ring mold for casting a hollow billet.
To the end, according to a first aspect of the present invention, there is provided a hot-top type continuous casting machine for casting a hollow billet, comprising: (a) a molten metal holding part into which molten metal is fed; (b) an ring mold arranged being connected to the molten metal holding part; and (c) a core inserted in the ring mold from the top of the later so as to define the inner wall of the hollow billet, and composed of an upper core which is low heat-conductive, and a lower core which is high heat-conductive and which is adapted to be cooled by cooling medium circulated therethrough.
According to a second aspect of the present invention, there is provided a hot-top type continuous casting machine in which the boundary part between the upper and lower cores is set at a heightwise position which is substantially equal to that of the ring mold.
According to a third aspect of the present invention, there is provided a hot-top type continuous casting machine in which both upper and lower cores are gradually tapered from the upper part to the lower part thereof with a predetermined gradient.
According to a fourth aspect of the present invention, there is provided a hot-top type continuous casting machine in which the high heat-conductive core is cooled by coolant circulating therethrough.
According to a fifth aspect of the present invention, there is provided a hot-top type continuous casting machine which is provided with a means for feeding lubrication oil onto the outer surface of the core in the boundary part between the low conductive upper core and the high conductive lower core.
According to a sixth aspect of the present invention, there is provided a hot-top type continuous casting machine in which the above-mentioned lubrication oil is rape seed oil or castor oil.
According to a seventh aspect of the present invention, there is provided a hot-top type continuous casting machine which is used for casting aluminum alloy.
According to an eighth aspect of the present invention, there is provided a core adapted to be used in a hot-top type continuous casting machine for casting a hollow billet, comprising (a) a low heat-conductive upper core and a high heat-conductive lower core; (b) a coolant supply pipe, and a coolant discharge pipe for discharging the coolant, which are extended through the core, (c) the coolant fed from the coolant supply pipe being discharged from the outside from the coolant discharge pipe after cooling the interior of the high heat-conductive core, and (d) means for feeding lubrication oil onto the outer surface of the core, being provided in the boundary part between the upper core and the lower core.
According to a ninth aspect of the present invention, there is provided a core used in a hot-top type continuous casting machine, in which the core is made of a heat-insulation board.
According to a tenth aspect of the present invention, there is provided a core used in a hot-top type continuous casting machine, in which the core is made of either aluminum alloy including AA6061 alloy, or copper alloy including Cuxe2x80x94Cr group alloy.
According to an eleventh aspect of the present invention, there is provided a core used in a hot-top type continuous casting machine, in which the coolant is water.
According to a twelfth aspect of the present invention, there is provided a core used in a hot-top type continuous casting machine, in which the lubrication oil is rape seed oil or castor oil.